Passing a Parameter to ViewModel constructor

Question

Is it possible to pass a parameter to the ViewModel constructor? I would then use this parameter to initialise a property and/or do other operations in the ViewModel.

With WinForms I could do

public MyForm(MyParamType myParam) {
    MyFormProperty = myParam;
    //etc.
}

How do I go about doing something similar in the MVVM pattern / using MVVM Light?

Any suggestions would be most welcome. Thanks in advance.

Answer 1

I would recommend using an IoC container and configuring your container to supply the parameter upon construction.

For instance, here's what a typical code-behind to a UserControl looks like for me in WPF:

public partial class MyDataGridView : IMyListView
{
    public MyDataGridView()
    {
      InitializeComponent();
    }

    public MyDataGridView(MyListViewModel viewModel)
    {
      InitializeComponent();

      DataContext = viewModel;
 }
}

StructureMap creates the MyListViewModel for me because by default it searches for the greediest constructor and then provides the dependencies. In my StructureMap configuration, I can specify that the MyListViewModel be provided with whatever parameters are necessary upon construction of that object.

With a container like StructureMap, I don't have to "new" up objects. Ever.

Answer 2

If you're using MVVM light (even if you're not I guess) you could register a message handler with the Messenger that takes your constructor parameters (or a tuple thereof) and updates the VM whenever you need to "reconstruct" it.

Answer 3

Since all the view models are static on the locator, you can just access those properties already without changing a constructor.

Answer 4

I dont understand why you cant just create the viewmodel yourself. You can always create your own viewmodel. If there is a viewmodel provided by MVVM Light then you can always inherit from that one and create an overloaded constructor.